OK...springs serve a few different functions. Ride height, rebound and to a MUCH lesser extent compression. You need enough spring to set the ride height where you need it and enough spring to reextend the suspension. The shock VALVING is what really controls how fast it compresses and how fast it rebounds. Srings do have a greater effect in controling body roll when turning.
Multiple rate systems have a heavier main spring and softer secondary springs. if you look at your works, there is an metal ring that floats on the shock inside the smaller spring. The height of the ring controls the crossover point. The softer spring will always compress first, once it hits the crossover point the softer spring will no longer compress and makes the bigger stiffer spring do all the work. So by changing crossovers you can change where the spring rate changes.
Now when you get into long travel situations, you can also use a really soft small spring, what that will do is let that soft spring compress to give you your ride height. this works when you are designing a lot of sag into the suspension. THis is called droop suspension, if you look at baja trucks on youtube, the truck itself seems to float while the wheels bounce all over the place, then when they catch air, the wheels fall away from the truck revealing huge wheel travel. All this allows the wheels to fllow the terrain bettter giving better traction and control while giving a smoother ride. Much different than the old stiff suspension to take the bike hits theroy.
Off road truck shocks are alot different than atv shocks. they run an atv type shock with springs on it to control ride height and rebound, but the main shock that does all the work is the bypass shock. Your shock works by forcing oil thru the valving. The bypass shock has very stiff valving bit it also has tubes welded to the outside with spring/check balls. each tube controls a specific part of the shocks travel, so you can have several inches around ride height that is really softly valved so the wheels can move easily over small bumps because the shock oil is bypassed thru the tube. Once you get passed the baypass the shocj gets much stiffer because you hit a bigger bump. Same with rebound. when you bottom out you want a slower rebound so you dont bounce as the shock extends, but on little bumps you want a fast rebound so the wheel follows the terrain.
See how low this thing sits?? then look at it in the air